There are many methods used to measure the depth of equipment within oil and gas wells. These usually consist of devices that are in physical contact with the conducting mechanism that transports the equipment to and from the surface through the wellbore.
In particular, when a coiled tubing operation is in progress, the prevailing method of depth measurement relies on one or more rotating measurement wheels that are in contact with the external surface of the coiled tubing. The measurement wheels are typically connected to quadrature encoders. The quadrature encoders produce a series of pulses as the measurement wheels rotate in concert with the linear motion of the coiled tubing as it passes by the measurement wheel. At any particular measurement wheel, the number of pulses is proportional to the length of tubing that has passed by the rotating measurement wheel. Those skilled in the art of coiled tubing operations will understand the normal process of feeding tubing into a wellbore.
Conventional depth measurement systems suffer accuracy and repeatability problems that manifest themselves as an inability to correctly record the depth of the leading end of the coiled tubing in the wellbore. Repeatability problems are often caused by an undetermined amount of slip that can vary as a result of changing conditions during the coiled tubing operation (e.g., ice on the coiled tubing, etc.). Slip results when tubing passes the measurement wheels without an equivalent rotation of the measurement wheels being recorded.
Further, repeatability errors may also arise due to the inability of existing depth measurement systems to measure the amount of stretch in the coiled tubing. Stretch is caused by the weight of the equipment attached to the tubing and the weight of the deployed tubing. The amount of tubing stretch also depends on other factors such as downhole friction and wellbore deviation.
Accuracy is dependent on factors such as the diameter of the measurement wheels being known to a specified tolerance. Measurement wheels are often prone to collecting dirt and grease which can result in a change in diameter. Although the diameter change may be small, over 10,000 feet of coiled tubing, the cumulative error in measurement due to a small change in diameter can be significant and undetected. Over a prolonged period of use, the surface of the measurement wheel begins to wear as a result of the rotating contact with the coiled tubing. This surface wear results in a decreased diameter of the rotating measuring wheel, which adds to depth measurement errors. There remains a need for improved measurement systems and methods.